Optical fiber communication system using remote pumping

ABSTRACT

An optical fiber communication system is provided which uses remote pumping that is capable of improving pumping efficiency and reducing a noise figure. A coupler ( 20 ) of a linear repeater ( 18 ) couples signal light to pumping light outputted from a pumping light source ( 19 ). The outputted signal light and pumping light reach a linear repeater ( 25 ) through transmission fibers ( 22  to  24 ) and remote pumping modules ( 27 F and  27 R). A coupler ( 30 ) of the linear repeater ( 25 ) couples the signal light to the pumping light supplied from a pumping light source ( 29 ), to output the signal light and the pumping light to the transmission fiber ( 24 ). The remote pumping module ( 27 F) divides the pumping light propagated in the transmission fiber ( 22 ), from the signal light. The remote pumping module  27 F branches the divided pumping light in two directions with a predetermined ratio. After branching, each of the branched pumping light is coupled to the signal light to be supplied to both ends of an erbium-doped fiber. The remote pumping module ( 27 R) is similar in structure to the remote pumping module ( 27 F).

TECHNICAL FIELD

This invention relates to an optical fiber communication system usingremote pumping for amplifying an optical signal by a passive remotepumping module which is apart from a linear repeater and a terminaldevice and which is installed in a constructed optical fiber which is atransmission path for the optical signal.

Priority is claimed on Japanese Patent Application No. 2003-271157,filed on Jul. 4, 2003, the contents of which are incorporated herein byreference.

BACKGROUND ART

FIG. 8 is a configuration example illustrating a conventional remotepumping system which is used in a wavelength division multiplexingoptical fiber communication system (referring to K. Aida et al., Proc.Of ECOC, PDA-7, pp. 29-32, 1989 and N. Ohkawa et al., IEICE Trans.Commun., Vol. E81-B, pp. 586-596, 1998). In this remote pumping system,signal light is transmitted from a transmitting circuit 2 of atransmitter 1 and is received by a reception circuit 11 of a receiver 10via three transmission fibers 5 to 7. Erbium-doped fibers (EDFs) 13F and13R are positioned between the transmission fiber 5 and the transmissionfiber 6 and between the transmission fiber 6 and the transmission fiber7, respectively. Remote pumping light sources 3 and 13 are located inthe transmitter 1 and receiver 10, respectively. The signal light iscoupled to the pumping light supplied from the pumping light source 3,by a coupler 4. Furthermore, the signal light is coupled to the pumpinglight supplied from the pumping light source 13, by a coupler 14. Eachof the transmitter 1, the receiver 10, and the pumping light sources 3and 13 is connected to power source to be supplied with a power. Thepumping light sources 3 and 13 adjacent to the transmitter 1 and thereceiver 10 will be called a front stage pumping light source and a rearstage pumping light source, respectively. In addition, the pumping lightsupplied from the pumping light sources 3 and 13 will be called aforward pumping light and a backward pumping light, respectively. Afterpassing through the transmission fiber 5, the forward pumping lightpumps the EDF 13F. After passing through the transmission fiber 7, thebackward pumping light pumps the EDF 13R.

Each pumping light has a wavelength near 1.48 μm which is appropriatefor pumping the EDF. The signal light outputted from the transmitter 1is attenuated in the transmission fiber 5 and is amplified in the EDF13F. Furthermore, the signal light is attenuated in the transmissionfiber 6 and is amplified in the EDF 13R. After passing through thetransmission fiber 7, the signal light is received by the receiver 10.Under such circumstances, it is possible to un-repeatedly transmit thesignal light over the entire length of the transmission fibers 5, 6, and7, without supplying power on the way. In comparison to a repeatingsystem which does not use the remotely pumped EDFs 13F and 13R, there isan advantage in greatly enlarging the un-repeated distance, namely,repeater spacing, in the above-mentioned remote pumping. Incidentally,it is possible to adopt either one of a configuration using the forwardpumping light source 3 and EDF 13F and a configuration using thebackward pumping light source 13 and EDF 13R. In addition, a certaindegree of distributed gain (Raman gain) is given to the signal light,since the signal light is generally subjected to Raman amplification inthe transmission fiber in which the pumping light is propagated.

In the conventional remote pumping system illustrated in FIG. 8, thepumping light, which reaches the EDFs 13F and 13R, travels from thepumping light input ends of EDFs 13F and 13R to the pumping light outputends of EDF 13F and 13R that are positioned at opposite sides of thepumping light input ends, since the gain wavelength region is set to Cband (1530 nm to 1560 nm) of EDFA in the signal light. As a result, thepumping light pumps the EDFs 13F and 13R across the entire fiberlengths, respectively.

However, it has been found that the pumping light, which reaches theEDFs 13F and 13R, only pumps to the vicinities of pumping light inputends of EDFs 13F and 13R and almost none travels to the opposite pumpinglight output ends, in the case where the gain wavelength region is setto L band (1570 nm to 1600 nm) of EDFA in the signal light. Since theEDF for the L band has a length which is several times as long as thelength of EDF for C band, the pumping light reaches only the vicinity ofthe pumping light input end. In addition, absorption occurs in the partof EDF that is not pumped, except in the vicinity of the pumping lightinput end. As a result, there is a problem in that the pumpingefficiency is reduced in each of the EDFs 13F and 13R and the noisefigure increases in each of the EDFs 13F and 13R. Incidentally, the Lband is a gain wavelength region for signal light and is as important asthe C band. More particularly, the L band is an important gainwavelength region for signal light since it is possible to prevent thefour-wave mixing, which is a problem in the C band, in a system using adispersion-shifted fiber (DSF).

DISCLOSURE OF INVENTION

This invention has been made taking the above-mentioned circumstancesinto consideration, and it is an object of this invention to provide anoptical fiber communication system using remote pumping that is capableof improving pumping efficiency and reducing a noise figure.

In order to solve the above-mentioned problems, a first aspect of thisinvention is an optical fiber communication system comprising: signallight output device which comprises a pumping light source which outputspumping light and a coupler which couples the pumping light to signallight; a plurality of transmission fibers which transmit the signallight outputted from the signal light output device; an erbium-dopedfiber module which is positioned between the transmission fibers; and asignal light reception device which receives the signal light which isoutputted from the signal light output device and which passes throughthe transmission fibers and the erbium-doped fiber module, wherein theerbium-doped fiber module comprises: a divider which divides the pumpinglight propagated in a direction the same as that in which the signallight is propagated in the transmission fibers, from the signal light; abranch which branches the pumping light divided by the divider in twodirection, at a predetermined ratio; an erbium-doped fiber to which thesignal light passing through the divider is inputted; and first andsecond coupling devices which couple the signal light to the pumpinglight branching off from the branch, to supply outputs of the first andthe second coupling device to both ends of the erbium-doped fiber.

A second aspect of this invention is an optical fiber communicationsystem comprising: a signal light output device which outputs signallight; a plurality of transmission fibers which transmit the signallight outputted from the signal light output device; an erbium-dopedfiber module which is positioned between the transmission fibers; and asignal light reception device which comprises: a pumping light sourcewhich outputs pumping light; and a coupler which couples the pumpinglight to the signal light which is outputted from the signal lightoutput device and which passes through the transmission fibers and theerbium-doped fiber module, to output the pumping light in a directionopposite to that in which the signal light is outputted, wherein theerbium-doped fiber module comprises: a divider which divides the pumpinglight propagated in the direction opposite to that in which the signallight is propagated in the transmission fibers, from the signal light; abranch which branches the pumping light divided by the divider in twodirections, with a predetermined ratio; an erbium-doped fiber to whichthe signal light is inputted; and first and second coupling deviceswhich couple the signal light to the pumping light branching off fromthe branch, to supply outputs of the first and the second couplingdevices to both ends of the erbium-doped fiber.

A third aspect of this invention is an optical fiber communicationsystem comprising: a signal light output device which comprises apumping light source which outputs pumping light and a coupler whichcouples the pumping light to signal light; a plurality of transmissionfibers which transmit the signal light outputted from the signal lightoutput device; an erbium-doped fiber module which is positioned betweenthe transmission fibers; and a signal light reception device whichreceives the signal light which is outputted from the signal lightoutput device and which passes through the transmission fibers and theerbium-doped fiber module, wherein the erbium-doped fiber modulecomprises: a circulator to which the signal light and the pumping lightare inputted; a first erbium-doped fiber to which the signal light andthe pumping light passing through the circulator are inputted; and amirror to which the signal light and the pumping light passing throughthe first erbium-doped fiber are inputted, and wherein the signal lightand the pumping light reflected by the mirror are outputted to a nextstage through the first erbium-doped fiber and the circulator.

In the third aspect of this invention, a second erbium-doped fiber maybe positioned at a front stage of the circulator.

A fourth aspect of this invention is an optical fiber communicationsystem comprising: a signal light output device which outputs signallight; a plurality of transmission fibers which transmits the signallight outputted from the signal light output device; an erbium-dopedfiber module which is positioned between the transmission fibers; and asignal light reception device which comprises: a pumping light sourcewhich outputs pumping light; and a coupler which couples the pumpinglight to the signal light which is outputted from the signal lightoutput device and which passes through the transmission fibers and theerbium-doped fiber module, to output the pumping light in a directionopposite to that in which the signal light is outputted, wherein theerbium-doped fiber module comprises: a circulator to which the signallight is inputted; a divider which divides the pumping light from thesignal light; a coupler which couples the pumping light divided by thedivider, to the signal light outputted from the circulator; a firsterbium-doped fiber to which the signal light and the pumping lightoutputted from the coupler are inputted; and a mirror to which thesignal light and the pumping light passing through the firsterbium-doped fiber are inputted, and wherein the signal light and thepumping light reflected by the mirror are outputted to a next stagethrough the first erbium-doped fiber and the circulator.

In the fourth aspect of this invention, a second erbium-doped fiber maybe positioned at a front stage of the circulator, and the coupler may bepositioned at a front stage of the second erbium-doped fiber.

According to this invention, it is possible to improve pumpingefficiency in the remote pumping module and to reduce a noise figure inthe remote pumping module in comparison to the conventional system,since the pumping light is inputted from both ends of the erbium-dopedfiber. Incidentally, this invention is not limited to application of theL band although this invention is of great value in the case where thegain wavelength region of the signal light is set to the L band of EDFA.This invention also has an effect when the gain wavelength region of thesignal light is set to, for example, C band of EDFA.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an opticalfiber communication system according to a first embodiment of thisinvention;

FIG. 2 is a block diagram illustrating a configuration of a remotepumping module 27F in the first embodiment of this invention;

FIG. 3 is a block diagram illustrating a configuration of a remotepumping module 27R in the first embodiment of this invention;

FIG. 4 is a block diagram illustrating a configuration of a remotepumping module 50F in an optical fiber communication system according toa second embodiment of this invention;

FIG. 5 is a block diagram illustrating a configuration of a remotepumping module 50R in the second embodiment of this invention;

FIG. 6 is a block diagram illustrating a configuration of a remotepumping module 70F in an optical fiber communication system according toa third embodiment of this invention;

FIG. 7 is a block diagram illustrating a configuration of a remotepumping module 70R in the third embodiment of this invention; and

FIG. 8 is a block diagram illustrating a configuration of a conventionaloptical fiber communication system.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be made as regards embodiments of this invention withreference to drawings hereinafter. FIG. 1 is a block diagramillustrating a configuration of an optical fiber communication systemaccording to a first embodiment of this invention. In FIG. 1, a linearrepeater 18 comprises a pumping light source 19 for generating pumpinglight, a coupler 20 for coupling the pumping light to signal light, andan isolator 21 for reducing multi-path interference noise. The signallight outputted from the linear repeater 18 reaches a downstream linearrepeater 25 via transmission fibers 22 to 24. A remote pumping module27F is located between the transmission fibers 22 and 23. A remotepumping module 27R is located between the transmission fibers 23 and 24.The linear repeater 25 comprises a pumping light source 29 forgenerating pumping light, a coupler 30 for coupling the pumping light tothe signal light, and an isolator 31.

Although an optical amplifier is installed in the linear repeater in aknown linear repeater system, no optical amplifier is installed in thelinear repeater in the present embodiment. In the present embodiment, itis possible to compensate for the sum of losses which occur in thetransmission fibers 22 to 24, since each of the remote pumping modules27F and 27R has a sufficiently large gain and the sum of distributedRaman gains is sufficiently large in the transmission fibers 22 and 24.Incidentally, the linear repeater 18 may be, for example, a transmitterand the linear repeater 25 may be, for example, a receiver. In otherwords, the linear repeater or the transmitter functions as signal lightoutputting means for outputting the signal light. The linear repeater orthe receiver functions as signal light reception means for receiving thesignal light.

FIG. 2 is a diagram illustrating a configuration of the remote pumpingmodule 27F. In the remote pumping module 27F, a divider 34 divides thepumping light propagated together with the signal light in a samedirection, from the signal light. A branching fiber coupler (FC) 35makes the divided pumping light branch in a predetermined branchingratio. In the example being illustrated, the branching ratio is one totwo. The pumping light of 33%, which is branched by the branching fibercoupler 35, is coupled to the signal light by a coupler 36 which isadjacent to the branching fiber coupler 35, to be inputted from aforward direction to an EDF 37. On the other hand, the pumping light of67%, which is branched by the branching fiber coupler 35, is suitablyattenuated by a variable attenuator (ATT) 38 which is adjacent to thebranching fiber coupler 35, to be inputted from a backward direction tothe EDF 37 by a circulator (CIR) 39 which is adjacent to the variableattenuator 38.

Incidentally, it is further preferable to use the circulator 39 althoughan optical coupler may be used instead of the circulator 39. Morespecifically, the circulator functions as aft optical coupler andisolator. Since the circulator interrupts returned light based on aresidual reflection, by the isolator function, it is possible to removemulti-path interference noise which occurs on the basis of the returnedlight, in a case where the returned light is not interrupted. Anisolator described hereinafter has the same function as described above.

FIG. 3 is a diagram illustrating a configuration of the remote pumpingmodule 27R. In the remote pumping module 27R, a divider 41 divides thepumping light transmitted to the module 27R in a direction opposite tothat in which the signal light is transmitted, from the signal light. Abranching fiber coupler 42 makes the divided pumping light branch in apredetermined branching ratio. In the example being illustrated, thebranching ratio is one to two. The pumping light of 67%, which isbranched by the branching fiber coupler 42, is coupled to the signallight by a coupler 43 which is adjacent to the branching fiber coupler42, to be inputted from a forward direction to the EDF 45. On the otherhand, the pumping light of 33%, which is branched by the branching fibercoupler 42, is suitably attenuated by a variable attenuator 46 which isadjacent to the branching fiber coupler 42, to be inputted from abackward direction to the EDF 45 by a circulator 47 which is adjacent tothe variable attenuator 46.

Since each of the EDFs 37 and 45 is supplied with the pumping light intwo directions with the predetermined branching ratio according to theconfigurations of the above-mentioned remote pumping modules 27F and27R, the pumping efficiency is enhanced in each of the EDFs 37 and 45and the noise figure is reduced in comparison to conventionaltechniques.

Incidentally, the variable attenuators 38 and 46 are for use inadjusting the branching ratio in the remote pumping modules 27F and 27R,respectively, in order to pump the EDFs 37 and 45 in accordance with thepredetermined branching ratios. Accordingly, it is possible to omit thevariable attenuators 38 and 45 when the branching ratios are known inadvance.

In addition, the above-mentioned branching ratio of 67% to 33% is anexample. The predetermined branching ratio is determined on the basis ofthe pumping efficiency and a noise property of each remote pumpingmodule. Each of the pumping efficiency and the noise property is one ofthe parameters which are used in determining a noise performance in theoptical fiber communication system. More specifically, the noiseproperty of the remote pumping module becomes better as the ratio of theforward direction increases and the pumping efficiency of the remotepumping module becomes better as the ratio of the backward directionincreases with respect to the branching ratio, when the input and outputdirections of signal light are determined as the forward and backwarddirections, respectively.

When installing the variable attenuators 38 and 46 as described above,it is possible to adjust the branching ratio in each remote pumpingmodule. As a result, there is an advantage in which it is sufficient toprepare the remote pumping modules similar to each other.

Next, a description of a second embodiment of this invention will begiven.

A system of the second embodiment is similar in configuration to thesystem of FIG. 1 except that remote pumping modules of the secondembodiment are different in structure from the remote pumping modules27F and 27R of the first embodiment. FIG. 4 is a diagram illustrating aconfiguration of a remote pumping module 50F which is positioned betweenthe transmission fibers 22 and 23 in FIG. 1. FIG. 5 is a diagramillustrating a configuration of a remote pumping module 5OR which ispositioned between the transmission fibers 23 and 24 in FIG. 1.

In the remote pumping module 50F illustrated in FIG. 4, the signal lightand the pumping light are inputted to an EDF 52 through first and secondports of a circulator 51. After passing through the EDF 52, the signallight and pumping light are reflected by a mirror 53 and pass throughthe EDF 52 in a direction opposite to the above-mentioned direction.After that, the signal light and pumping light pass through a third portof the circulator 51 and are outputted from the module.

In the remote pumping module 5OR illustrated in FIG. 5, a divider 55divides the pumping light transmitted to the module 50R in a directionopposite to that in which the signal light is transmitted, from thesignal light. The divided pumping light is inputted to a coupler 57which is adjacent to an EDF 56. The signal light passes through a secondport of a circulator 58 and is coupled to the above-mentioned pumpinglight in the coupler 57. The signal light and the pumping light, whichare outputted from the coupler 57, are inputted to the EDF 56. Afterpassing through the EDF 56, the signal light and the pumping light arereflected by a mirror 59 and pass through the EDF 56 in a directionopposite to the above-mentioned direction. After that, the signal lightand the pumping light pass through a third port of the circulator 56 andare outputted from the module through the divider 55.

According to the configurations of the above-mentioned remote pumpingmodules 50F and 50R, it is possible to construct each of the moduleswith a decreasing number of parts in comparison to the first embodiment.Since the second embodiment uses a double path configuration having ahigh pumping efficiency, with respect to the signal light, it ispossible to obtain a high pumping efficiency which is not less than thepumping efficiency of the first embodiment.

Next, a description of a third embodiment of this invention will begiven.

A system of the third embodiment is similar in configuration to thesystem of FIG. 1 except that remote pumping modules of the thirdembodiment are different in structure from the remote pumping modules27F and 27R of the first embodiment. FIG. 6 is a diagram illustrating aconfiguration of a remote pumping module 70F which is positioned betweenthe transmission fibers 22 and 23 in FIG. 1. FIG. 7 is a diagramillustrating a configuration of a remote pumping module 70R which ispositioned between the transmission fibers 23 and 24 in FIG. 1.

The remote pumping module 70F illustrated in FIG. 6 is different fromthe remote pumping module 50F (FIG. 4) of the second embodiment inpositioning an EDF 71 in a front stage of the circulator 51 andamplifying the signal light before carrying out amplification by the EDF52.

In addition, a remote pumping module 70R illustrated in FIG. 7 isdifferent from the remote pumping module 50R (FIG. 5) of the secondembodiment in positioning an EDF 72 in a front stage of the circulator58 and amplifying the signal light before carrying out amplification bythe EDF 56. In the example being illustrated, the coupler 57 ispositioned in front of the EDF 72 and the pumping light is coupled tothe signal light by the coupler 57. The coupled signal light and pumpinglight are inputted to the EDFs 72 and 56 and are reflected by the mirror59. The signal light and the pumping light are outputted from the modulethrough the circulator 58 and the divider 55.

Incidentally, the EDF 52 according to the third embodiment may have alength that is less than that of the EDF 52 according to the secondembodiment by the length of the EDF 71. Similarly, the EDF 56 accordingto the third embodiment may have a length that is less than that of theEDF 56 according to the second embodiment by the length of the EDF 72.

The EDF module of a so-called double path type which is described in thesecond embodiment using the circulator and the mirror has a defect inwhich the noise figure increases in the module because the signal lightinput end corresponds to the signal light output end in the EDF adjacentto the circulator. In other words, the power of signal light reaches ahigh level which is approximately equal to the level of pumping light,by amplifying the signal light. As a result, this parameter deteriorateswith respect to a population inversion and the noise becomes large. Onthe other hand, in the third embodiment, the EDFs 71 and 72 carry outfront stage amplifications to suppress the increase of noise figures inthe EDFs 52 and 56, respectively. As a result, according to the thirdembodiment, it is possible to reduce the noise figure in comparison tothe second embodiment.

Although description is made as regards each of the embodiments of thisinvention, this invention is not limited to the-above-mentionedembodiments and it is possible to make additions, omissions,replacements, and various changes within the sprit and scope of thisinvention.

INDUSTRIAL APPLICABILITY

This invention is for use in an optical fiber communication system foramplifying an optical signal in accordance with a remote pumping carriedout by a passive remote pumping module which is apart from a linearrepeater and a terminal device. This invention is of great value for usein a system in which the gain wavelength region of signal light is setto the L band of EDFA. For example, this invention is suitable for asystem using a DSF for preventing four wave mixing, which is a problemoccurring in the C band of EDFA. According to this invention, it ispossible to improve pumping efficiency in the remote pumping module andto reduce a noise figure in the remote pumping module, since the pumpinglight is inputted from both ends of the erbium-doped fiber.

1. An optical fiber communication system comprising: a signal lightoutput device which comprises a pumping light source which outputspumping light and a coupler which couples the pumping light to signallight; a plurality of transmission fibers which transmit the signallight outputted from the signal light output device; an erbium-dopedfiber module which is positioned between the transmission fibers; and asignal light reception device which receives the signal light which isoutputted from the signal light output device and which passes throughthe transmission fibers and the erbium-doped fiber module, wherein theerbium-doped fiber module comprises: a divider which divides the pumpinglight propagated in a direction the same as that in which the signallight is propagated in the transmission fibers, from the signal light; abranch which branches the pumping light divided by the divider in twodirection, at a predetermined ratio; an erbium-doped fiber to which thesignal light passing through the divider is inputted; and first andsecond coupling devices which couple the signal light to the pumpinglight branching off from the branch, to supply outputs of the first andthe second coupling device to both ends of the erbium-doped fiber.
 2. Anoptical fiber communication system comprising: a signal light outputdevice which outputs signal light; a plurality of transmission fiberswhich transmit the signal light outputted from the signal light outputdevice; an erbium-doped fiber module which is positioned between thetransmission fibers; and a signal light reception device whichcomprises: a pumping light source which outputs pumping light; and acoupler which couples the pumping light to the signal light which isoutputted from the signal light output device and which passes throughthe transmission fibers and the erbium-doped fiber module, to output thepumping light in a direction opposite to that in which the signal lightis outputted, wherein the erbium-doped fiber module comprises: a dividerwhich divides the pumping light propagated in the direction opposite tothat in which the signal light is propagated in the transmission fibers,from the signal light; a branch which branches the pumping light dividedby the divider in two directions, with a predetermined ratio; anerbium-doped fiber to which the signal light is inputted; and first andsecond coupling devices which couple the signal light to the pumpinglight branching off from the branch, to supply outputs of the first andthe second coupling devices to both ends of the erbium-doped fiber. 3.An optical fiber communication system comprising: a signal light outputdevice which comprises a pumping light source which outputs pumpinglight and a coupler which couples the pumping light to signal light; aplurality of transmission fibers which transmit the signal lightoutputted from the signal light output device; an erbium-doped fibermodule which is positioned between the transmission fibers; and a signallight reception device which receives the signal light which isoutputted from the signal light output device and which passes throughthe transmission fibers and the erbium-doped fiber module, wherein theerbium-doped fiber module comprises: a circulator to which the signallight and the pumping light are inputted; a first erbium-doped fiber towhich the signal light and the pumping light passing through thecirculator are inputted; and a mirror to which the signal light and thepumping light passing through the first erbium-doped fiber are inputted,and wherein the signal light and the pumping light reflected by themirror are outputted to a next stage through the first erbium-dopedfiber and the circulator.
 4. An optical fiber communication system asclaimed in claim 3, further comprising a second erbium-doped fiber whichis positioned at a front stage of the circulator.
 5. An optical fibercommunication system comprising: a signal light output device whichoutputs signal light; a plurality of transmission fibers which transmitsthe signal light outputted from the signal light output device; anerbium-doped fiber module which is positioned between the transmissionfibers; and a signal light reception device which comprises: a pumpinglight source which outputs pumping light; and a coupler which couplesthe pumping light to the signal light which is outputted from the signallight output device and which passes through the transmission fibers andthe erbium-doped fiber module, to output the pumping light in adirection opposite to that in which the signal light is outputted,wherein the erbium-doped fiber module comprises: a circulator to whichthe signal light is inputted; a divider which divides the pumping lightfrom the signal light; a coupler which couples the pumping light dividedby the divider, to the signal light outputted from the circulator; afirst erbium-doped fiber to which the signal light and the pumping lightoutputted from the coupler are inputted; and a mirror to which thesignal light and the pumping light passing through the firsterbium-doped fiber are inputted, and wherein the signal light and thepumping light reflected by the mirror are outputted to a next stagethrough the first erbium-doped fiber and the circulator.
 6. An opticalfiber communication system as claimed in claim 5, further comprising asecond erbium-doped fiber which is positioned at a front stage of thecirculator, wherein the coupler is positioned at a front stage of thesecond erbium-doped fiber.